The term has been around since the beginning of 2017 as Bitcoin entered its third cycle of growth and the price started to reach absurd amounts in fiat unit prices. As the price drew much more attention, many standards wondered how Bitcoin could be valued at several thousand dollars per coin.
Now, many Bitcoin educators would focus on the supply shortage due to the halving cycle and growing integration, but these concepts are quite difficult to distill into a single tweet. Hence the number of oversimplifications or NGUs.
Each halving cycle (every four years) brings supply and, in theory, would push the price up even if demand remains constant. This is why many Bitcoiners believe in its NGU technology which is built into the protocol.